In a complaint filed this afternoon with the Federal Election Commission, the Democratic National Committee said that McCain had pledged future public election funds as collateral for a loan the campaign took out last year. The Democrats say that prevents the Arizona senator from withdrawing from the public funding programme – and the spending limits it imposes.

The filing accuses McCain of failing to report that he pledged as collateral future public funds, known as matching funds because they double some small private campaign donations.

It's unclear what, if any, effect the filing will have. The six-member commission currently has four vacancies. That's too few to constitute a quorum and initiate enforcement actions.

The four vacant seats are the subject of an ongoing battle between the Democrat-controlled Senate and the Republican White House.

One senator blocking the nomination is Barack Obama. In part due to his objection, the Senate has refused to confirm one nominee, Hans von Spakovsky, accusing him of blocking voter discrimination cases while a justice department lawyer. In response, Senate Republicans have refused to allow confirmation of Bush's three other appointees.

If McCain is found to have violated election law, by breaking provisions of the public matching funds programme, it's unclear whether he'll face significant penalties.

"The reality is, this is more of a PR problem than a substantive problem for McCain," said former Republican federal election commissioner Bradley Smith.

The McCain campaign listed the loan in a January finance report with the Federal Election Commission, but wrote that McCain put up his own assets and future earnings as collateral rather than the public funds.

The Democratic party warned that McCain is poised to surpass the spending limit imposed by the public funds programme.

McCain has cast himself as a reformer on campaign finance, and the Monday complaint is part of an early effort by Democrats to counter that image ahead of the general election campaign. Having outlasted every Republican candidate but former Arkansas governor Mike Huckabee, McCain is almost certain to be his party's nominee for president.

McCain "used taxpayer money to guarantee a loan so he could raise money from lobbyists and special interests," Democratic National Committee chairman Howard Dean said in a statement announcing the filing. "It's the height of hypocrisy."

Along with liberal Democratic senator Russ Feingold of Wisconsin, McCain was a prime sponsor of a 2002 effort to rein in the power of corporate and union money in federal elections. That earned McCain enmity on the right, which considers such attempts an infringement on the freedom of speech.

The public matching funds programme is intended to permit less well-heeled candidates to campaign for president. Candidates who participate may spend no more than about $50 million, and must adhere to spending limits in each state. This year most candidates have declined to join the programme for fear the spending limits would hinder their efforts. As of the end of January, McCain had spent roughly $49 million, according to campaign finance reports.

Among leading Democrats, only John Edwards participated in the programme; McCain was the only top Republican. He signed up in August, when his campaign was faltering and running desperately short of money.

In November, McCain received a $4m loan from a local bank to fund his campaign. In a year-end campaign finance report, his camp specifically notes that public funds are not pledged as collateral.

But according to the Democratic complaint, the wording of the loan agreement suggests the opposite. That document includes language the party says asserts the bank's interest in McCain's public matching funds.

Following McCain's January 3 victory in New Hampshire donations began flowing more readily, and earlier this month McCain told the Federal Election Commission his campaign was withdrawing from the programme. The campaign never actually received any public money. The Democratic party argues that participation in the public matching funds programme is binding and that McCain can't be released from the programme without the commission's consent. It argues further the commission can't release him from the programme because he has pledged the public funds as collateral for the loan.

The Democrats argue further that McCain unduly benefited from his ostensible initial participation in the program. Election law in Ohio and other states allow candidates who are eligible for matching funds to skip costly and time-consuming signature-gathering to gain access to the ballot.

If the commission is reconstituted, it will have to decide whether McCain did in fact promise public matching funds to back the loan.

If so, the act would bind him to the programme's spending limits, which he is set to breach imminently. The penalty for violating the spending limits is repaying federal matching funds – that likely will not be a problem for McCain, because he has said he won't accept any.

If the commission decides the violation was "knowing and willing", it could fine and even imprison campaign officials, but that's unlikely, Smith, the former federal election commissioner, said.

The McCain campaign did not immediately respond to a request for comment.